In zoom lenses for home video cameras which have recently been widely popularized, simplification of the zoom lens construction is desired because of market demands for smaller size, less weight, and lower pricing. A zoom lens suitable for a home video camera has been disclosed in Japanese Laid Open Application H4-153615 wherein, in order from the object side, the first lens group has positive refractive power and is fixed in position, the second lens group has negative refractive power and is moved in the direction of the optical axis to change the focal length during zooming, the third lens group has positive refractive power and is fixed in position, and the fourth lens group has positive refractive power and is moved along the optical axis to change the focal length during zooming and to correct what would otherwise be a fluctuation of image surface position with movement of the second lens group or a change of object distance. In zoom lenses of this type, movement of the fourth lens group for both zooming and focusing is an important factor that enables such zoom lenses to have a simplified construction. Moreover, in this type of zoom lens, the lens groups also are smaller and lighter in weight due to miniaturization of the zoom lens. Further, the movement of the moveable lens groups during zooming or focusing is easy. Thus, this type of construction has been both convenient and popular.
By contrast, in video cameras used for broadcasting or other business uses, higher imaging performance has been demanded than is provided by zoom lenses used for home video cameras. Thus, the picture quality provided by broadcasting cameras is higher than that of video cameras for home use, especially in the case of recording color images. However, it is difficult to provide images having sufficient quality using simplified-construction zoom lenses as found in home video cameras.
As an example of a high-performance zoom lens suitable for broadcast quality imaging, a zoom lens has been disclosed in Japanese Laid Open Patent Application H3-56915. This zoom lens is formed of five lens groups having refractive powers of, in order from the object side, positive, negative, negative, positive, and positive, respectively. The focal length of the zoom lens is changed by moving the second and third lens groups along the optical axis when zooming, and focusing is performed by moving, for example, all or some of the lens elements of the first lens group. Thus, the lens elements moved for focus adjustment are moved independently of the lens elements that are moved for varying the focal length during zooming.
Manual focus and auto focus are known focusing techniques for zoom lenses. Auto focus (for example, as described in Japanese Patent 2569093 and in Japanese Laid Open Patent Application H11-2757) has been used thus far primarily in zoom lenses for home video cameras, as these video cameras emphasize ease of operation. These zoom lenses are constructed so that the direction and amount of movement of the lens elements for focusing are automatically determined based on images received by an image pickup array that is used to perform auto focus. Namely, in a zoom lens for home video cameras, it is common for a portion of the optical system in the zoom lens to be micro-vibrated in the direction of the optical axis, while the micro-vibrated portion is automatically moved in the direction of the optical axis for focus adjustment in order to match the best image surface position with the position of an image pickup array. The best match is based on a signal extracted from an image signal detected by an image pickup array of the zoom lens. Thus, a zoom lens of simple constitution is achieved by moving the micro-vibrated optical system in order to accomplish the focusing.
The zoom lens described in Japanese Patent No. 2569093 comprises a fixed front lens group, a variator lens group which varies the, focal length, a compensator lens group for correcting for what would otherwise be aberrations caused by varying the focal length, and a main lens group. Some or all of the lens elements of the main lens group are made so as to be both shakable and movable by using the step driving of a pulse motor, and these lenses are moved by the step driving of the pulse motor based on a signal extracted from an image signal obtained from an image pickup array while micro-vibrating these lenses in the direction of the optical axis.
The zoom lens described in Japanese Laid Open Patent Application H11-2757 comprises a fixed front lens group, a variator lens group, a diaphragm, a fixed lens group, a micro-vibrating portion and an imaging portion. The micro-vibrating portion functions both as a focusing portion and a correcting portion. If a controlling means micro-vibrates the micro-vibrating portion in the direction of the optical axis via a driving means, a focus evaluating means evaluates the focusing condition of formed photo images for each frame, an operating means determines the direction and amount of movement of the micro-vibrating portion based on these evaluated values, and then the driving means drives the micro-vibrating portion in the direction of the optical axis to match the focus based on these values.
By contrast, it is sometimes desired to express subtle nuances while recording various scenes using high-performance video cameras, e.g., those of broadcast quality. Therefore, a manually focused zoom lens having a high degree of freedom and that is capable of performing focus or zoom manually based on visual information of finder images has been desired by professional cameramen.
Recently, market demand has arisen for auto focus in video cameras of broadcasting and business quality. For example, in news gathering the quickness of reports is emphasized to such an extent that there is a need to shorten the time required by a professional cameraman to focus a scene. Or, if the depth of subjects is too shallow to be focused like a high-performance camera, it takes time for a subtle adjustment even by a skilled cameraman. Moreover, sometimes automatic focusing by a high-precision camera is needed beyond a level of focus adjustment that can be obtained using one""s eyes.
There is a problem in applying the auto focus mechanism of zoom lenses of home video cameras to zoom lenses of high-performance broadcast or business-use cameras. Namely, the lens elements of zoom lenses for home video cameras have tended to become small-sized and light-weight due to the miniaturization of the zoom lens with the simplification of design. Accordingly, the lens groups that are moved for focusing are light-weight, and the micro-vibration of these lenses in the direction of the optical axis to obtain information concerning the optimum focus position has not been a problem. However, the lens elements that move for focusing of zoom lenses used for broadcasting and business tend to be large and heavy. The micro-vibration of these lens elements is not realistic. Therefore, market demand for a zoom lens which has a different construction from that of state-of-the-art home video cameras and is well-suited for use in a high-performance video camera of broadcast or business quality, and which has auto focus, has intensified.
Although zoom lenses with auto focus operation are the norm for home video cameras where ease of operation is paramount, for broadcast or business quality video cameras manual operation mode is the norm and the use of auto focus is supplemental. Situations requiring the use of auto focus are not all that frequent for a professional cameraman. Nevertheless, a high quality zoom lens which can operate in auto focus mode is needed for those situations when a cameraman feels that manual operation mode of the zoom lens is not satisfactory.
A first object of the invention is to provide a high-performance zoom lens suitable for business video cameras in which focus may be selectively adjusted manually when in manual mode, or automatically when in auto focus mode. A second object of the invention is to provide a zoom lens capable of auto focus even where the lens elements that are moved during manual focus adjustment are large and heavy.